The invention relates to a laser arrangement comprising a pump unit containing a pumped laser crystal, and further comprising means for passive mode-locking.
Such laser arrangements are particularly provided as short pulse laser arrangements, wherein short laser pulses are generated with high energy in the mode-locked state. These short pulse laser arrangements are advantageously used for high-precision material processing or for scientific tasks. As regards the generation of short laser pulses, reference may be made, e.g., to the general statements in WO 98/10494 A as well as in A. Stingl et al.: Generation of 11-fs pulses from a Ti:sapphire laser without the use of prisms; Optics Letters Vol. 19, No. 3, 1 Feb. 1994, pp. 204-206.
The standard technique for generating short laser pulses with high energy is based on the technique of a laser oscillator and a laser amplifier. The laser oscillator generates a sequence of short laser pulses of low energy, e.g. with a repetition frequency in the range of some ten Mhz. From these oscillator pulses, pulses with a lower repetition frequency are selected and amplified in a regenerative or so-called multi-pass amplifier to give pulses of high energy.
An all-in-one concept is preferably used, in which merely one laser is provided which is used both as osciliator and also as regenerative amplifier, by simply performing the pulse formation (at low energies) on the one hand, and the amplification (to high energies), on the other hand, at different times. Known laser arrangements of this type (cf. e.g. L. Turi, T. Juhasz: Diode-pumped Nd:YLF all-in-one laser; Optics Letters Vol. 20, No. 14, 15 Jul. 1995, pp. 1541-1543), comprising a laser both for the oscillator function and also for the amplifier function, use active mode-locking with an acousto-optic modulator. Since the active mode-locking is not very efficient, the minimum pulse duration obtainable is relatively long, and moreover, a complex electronic circuitry is required for the time control and stabilization, respectively, to achieve a reliable long-term operation.
On the other hand, in laser arrangements it is generally knownxe2x80x94as results from the previously mentioned document WO 98/10494 A, to design a passive mode-locking, in particular also with a saturable absorber. Other possible ways for passive mode-locking are, e.g., the utilization of the Kerr effect (so-called Kerr lens mode-locking, KLM), the use of non-linear mirrors (non-linear mirror mode-locking, NLM), the application of a non-linear polarization rotation in optic crystals or cascaded non-linear processes of the second order.
It is now an object of the invention to provide a laser arrangement of the initially defined type which, when using a passive mode-locking, allows for an all-in-one construction of the laser, wherein the advantage is used that a passive mode-locking causes a much stronger amplitude modulation than an active mode-locking and stable short pulses in the pico- or femto-second range can be generated, i.e. with pulse durations near the possible lower limit value, which is given by the finite amplification bandwidth of the laser material used. Here, also the problem must be solved that passive mode-locking devices, such as saturable absorbers, on the other hand provide an upper limit to the raising of the energy, due to their low destruction threshold value, and moreover, with an excessive saturation of the absorber, also instabilities are caused. The invention now is based on the idea to distribute the resonator to two different sub-resonators active at different times, with the sub-resonators providing the different tasks, i.e. the pulse formation by using the passive mode-locking, on the one hand, with high resonator losses corresponding to the low power levels, and, on the other hand, the amplification to high pulse energies.
The inventive laser arrangement of the initially defined type therefore is characterized in that two separate, alternatively switchable resonator arms are provided, one of which, which is active in the one pulse forming phase, comprises the means for passive mode-locking, whereas the other resonator arm, which is active in an amplifying phase, is free from components that introduce losses. In the present laser arrangement, thus, different resonator parts are used at different times, wherein in the one phase, when the short laser pulses having a low energy are generated, the one resonator arm is active with the arrangement for passive mode-locking, with high resonator losses corresponding to the low power values prevailing; after the pulse formation, the resonator is switched over so that the other resonator arm becomes active, wherein the means for passive mode-locking is no longer present in the resonator, and an amplification to high pulse energies becomes possible with a view to the high resonator quality of the resonator now active.
Suitably, the polarization of the laser beam is utilized for switching, and accordingly, for a simple embodiment of the laser arrangement it may be provided that for switching between the two resonator arms, at least one polarization-sensitive beam divider as well as a polarization rotating means is provided. To this end, preferably a Pockels cell is used as the polarization rotating means; such a Pockels cell may be controlled electrically so as to rotate the polarization direction of the laser beam passing therethroughxe2x80x94by 90xc2x0. In combination with another appropriate polarization-sensitive beam divider, thus, the laser beam can be directed into the other resonator arm after this switching.
It is particularly advantageous if one polarization-sensitive beam divider each is arranged in the path of the laser beam on either side of the polarization rotating means. In this instance, the polarization-sensitive beam divider provided on the side of the polarization-rotating means facing away from the means for passive mode-locking may form a laser beam out-coupling element so as to couple out the amplified laser pulses.
As a means for passive mode-locking, preferably a saturable absorber is used, as is known per se.
The saturable absorber advantageously may be a per se known saturable semiconductor absorber.
Moreover, for a compact, efficient resonator embodiment it is also advantageous if the saturable absorber is an absorber mirror terminating the one resonator arm, which absorber mirror in particular is semiconductor-based.
To introduce the desired losses in the laser beam during the pulse forming phase, it is also suitable to provide a linear loss element, e.g. a xcex/4 platelett, in the one resonator arm which is active in the pulse forming phase, which platelett provides for a high energy accumulation in the laser crystal.
For designing the laser arrangement as an all-in-one laser system, finally, it is also suitable if the pumping unit is a continuous wave diode pumping unit and, together with the polarization rotating means, forms a resonator part common to both resonator arms. Moreover, it is advantageous if the pumping unit is lamp-pumped or laser-pumped and, together with the polarization rotating means, forms a resonator part common to both resonator arms.